The present invention relates to nuclear fuel assemblies, and more particularly, to lateral load-resistant grids for supporting fuel rods within nuclear fuel assemblies of light water nuclear reactors.
U.S. Pat. No. 4,897,241 issued Jan. 30, 1990 to Anthony, entitled "Anti-Bow Grid For Nuclear Fuel Assembly", discloses an improved grid having a plurality of externally projecting, integrally formed anti-bowing springs spaced about the grid perimeter plate, for interacting with flat surfaces on opposed grids of adjacent fuel assemblies in the reactor core. The anti-bowing springs are horizontally separated along each grid in periodic alternation with substantially flat portions of the perimeter plate. Preferably, each grid has a plurality of primary anti-bowing springs that project a distance at least equal to the nominal gap between assemblies, and a plurality of secondary anti-bowing springs that are stiffer but project less than the primary springs. Further, back-up springs may optionally be provided. The interaction of each grid with its neighbor "tightens" the core during power operation and thereby inhibits bowing of one or more fuel assemblies in the core.
Resistance to bowing, however, is but one of a number of desirable performance characteristics of nuclear fuel assemblies relative to externally imposed lateral forces or loads. Such loads can arise during postulated seismic disturbances or loss of coolant accidents. Furthermore, localized contact between assemblies during normal operation can induce friction-induced wear.
Although vendors of nuclear steam supply systems have the opportunity to select materials and configurations for fuel assemblies by taking advantage of trade-offs with available margins in other core internals and performance parameters, such flexibility is not always available for suppliers of aftermarket reload fuel assemblies to be used with core internals and performance parameters originally designed by others. In designing reload fuel assemblies, it is usually preferable, if not necessary, to match or exceed coolant flow rates and strength characteristics of the grids on the original vendor's fuel assemblies. This objective is more difficult to achieve when the grids on the reload fuel assemblies are to be made of a material different from the grids of the original assemblies. In particular, if a reload fuel supplier desires to utilize grids made of Zircaloy, which has the advantage of relatively low parasitic absorption of neutrons in the core, the designer must compensate for the relatively weak structural strength of Zircaloy as compared to Inconel-718, which in the past has often been used as grid material in original fuel assemblies. Therefore, in order to match grid strength, conventional thinking dictates that the Zircaloy grid strips must be thicker or the intersection strip welds must be larger, both of which would cause higher pressure drops and mis-matched flow characteristics. Mis-matched flow characteristics may cause flow induced vibrations which could lead to mechanical failure due to fuel rod fretting, or thermal or heat transfer distribution problems.
Although Zircaloy grids with the optional back up springs described in the Anthony patent mentioned above can help prevent fuel assembly damage during a seismic event, the back up springs are disclosed only in the context of primary and secondary springs which have first "tightened up" the core.